The pilot reported he departed the airstrip and experienced light turbulence. He entered a left traffic pattern to the downwind leg and the turbulence stopped. After executing a touch-and-go landing the pilot departed to the south. Subsequently, between 50 and 100 feet above ground level (AGL) he began to lose lift. The pilot landed the airplane in a rice field and it nosed over as the wheels contacted the vegetation. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.